Method and apparatus for removing water from a natural gas well

ABSTRACT

Water is removed from a natural gas well by the use of nitrogen, which is produced by a non-cryogenic unit at the site of the well. A cylindrical casing is positioned over the well. A tubing, disposed within the casing, is aligned over the well bore. Nitrogen is selectively introduced into either the space between the tubing and the casing, or into the tubing, so as to displace water from within the tubing or the casing, respectively. The water is directed to a storage tank at the site of the well. Nitrogen is purged from the lines, and the well is ready to resume production.

CROSS-REFERENCE TO PRIOR APPLICATION

Priority is claimed from U.S. Provisional Patent Application Ser. No. 61/441,348, filed Feb. 10, 2011, the disclosure of which is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

The present invention relates to the dewatering of a well, especially a natural gas well.

A natural gas well eventually becomes filled with water, which leaches into the well from surrounding underground water sources. The rate of intrusion of water varies according to the conditions of the particular well, but after the passage of sufficient time, virtually any well will become filled with water. The water must be removed in order to continue to extract natural gas efficiently from the well.

The current method of dewatering a natural gas well consists of using a piece of equipment called a “swab rig”. This equipment includes a truck-mounted derrick, and associated tooling, to remove water mechanically from the well. The water is removed by means of a cable with cups attached to the cable. The process consists of lowering the cable, with the attached cups, into the well, allowing the liquid in the well to fill the cups, and removing the water from the well by retrieving the cable and disposing of the liquid.

The above-described method is limited by the amount of water that can be held in the total volume of the cups, which is usually no more than about three barrels altogether. The process of dewatering can be very time-consuming, because to remove most or all of the water, the process must usually be repeated multiple times. This involves lowering the cable again into the well. The deeper the well, the longer it takes between each trip of the cable with the cups. This process of completely dewatering a well, so that the well can again be used reliably to produce natural gas, using this prior-art method, can take hours to weeks for each well. Moreover, if some gas flows out of the well between runs, it becomes necessary to wait until the pressure bleeds off before another cable run can be made, thus adding to the total time required for dewatering.

In practice, many wells may be 10,000 feet deep, and a typical well may have an amount of water in the range of about 20-50 barrels. Some wells may have as little as five barrels of water, and others could have as much as 200-300 barrels or more. For a swabbing unit to remove this water, it might be necessary to make of the order of 7-15 runs, or more, of the cable.

The above-described method is also very prone to environmental spills, as the fluid being removed from the well must be transferred to a storage vessel for disposal.

The present invention provides a method of dewatering which has much greater efficiency, and works in a dramatically shorter time frame, than the methods of the prior art.

SUMMARY OF THE INVENTION

The present invention, in brief, comprises a process which includes using an on-site nitrogen generator that can deliver enough nitrogen to force water out of the well. More particularly, the nitrogen generator delivers nitrogen at high pressure, at a rate of up to 1000 scfm at 99% purity and 4000 psi. The high-pressure nitrogen gas is injected down the well tubing, forcing the water out of the tubing and into the casing which surrounds the tubing. The fluid is then pushed up the well piping and is collected in an on-site tank.

More particularly, in a preferred embodiment, nitrogen is first directed into the tubing positioned substantially in the center of the well bore. The tubing defines a central region, within the tubing, and an outer region, outside the tubing and within the well casing, which is disposed in the well bore, and which extends above the well bore. The nitrogen flows down into the tubing, and then flows up the casing, thereby clearing fluid and sand from the well bore. Then, nitrogen is directed into the casing, so that it is forced to flow upward through the tubing, thereby lifting any remaining fluid in the tubing. Residual nitrogen is then purged from the lines, and the well is again ready to produce natural gas.

The average time spent in performing the process of the present invention is only about three hours.

The present invention therefore has the primary object of providing a method of removing water from a well, especially a natural gas well.

The invention has the further object of reducing the time needed to remove water from a well.

The invention has the further object of providing a method as described above, wherein the method is not prone to environmental spills.

The invention has the further object of improving the safety of the process of removing water from natural gas wells.

The invention has the further object of improving the efficiency of production of natural gas.

The invention has the further object of providing apparatus for performing the above-described method.

The reader skilled in the art will recognize other objects and advantages of the present invention, from a reading of the following brief description of the drawings, the detailed description of the invention, and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 provides a schematic diagram of a well dewatering system made according to the present invention.

FIG. 2 provides a table showing valve positions, for the valves of FIG. 1, at various stages of the process of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 provides a schematic diagram of the system of the present invention. FIG. 1 should be read in conjunction with FIG. 2, which is a table showing the valve configurations for each stage of the process.

As shown in FIG. 1, a generally cylindrical casing 100 is inserted into the well bore, and extends above the surface 110 at the site of the well. A cylindrical tubing 101 is located within the casing. The tubing is aligned with the well bore 107. In this specification, the space that is within the casing, but outside the tubing, will be identified, for convenience of description, as the “casing”. Similarly, the space that is within the tubing is designated as the “tubing”. Nitrogen generator 102 is located near the well head, and provides nitrogen for the method of the present invention. The nitrogen generator is preferably a membrane unit. That is, the nitrogen generator contains one or more modules having polymeric membranes which separate air into components. The technology of the membrane unit is known in the art, and does not form part of the present disclosure.

Examples of membrane-based air separation systems, and/or modules of fiber membranes, are given in the following U.S. patents, the disclosures of which are all incorporated by reference herein: U.S. Pat. Nos. 4,881,953, 4,900,334, 4,955,993, 5,141,530, 5,163,977, 6,136,073, 7,497,894, 7,517,388, 7,578,871, and 7,662,333.

The system shown in FIG. 1 also includes waste water storage tank 103 and separation unit 104. The purpose of the separation unit is to remove water from the product stream of natural gas, as the gas exits the well. The separated water can be conveyed to the waste water storage tank, through conduit 105.

In broad outline, the method of the present invention comprises injecting nitrogen into the well so as to displace and remove water from the well. More specifically, continuous injection of gas into the casing causes the gas to flow upward in the tubing, thereby removing water from the tubing. Similarly, continuous injection of gas into the tubing causes the gas to flow upward in the casing, thereby removing water from the casing.

A more specific procedure for accomplishing the above is explained in more detail below. This procedure is summarized by the column headings of FIG. 2, which shows the positions of the various valves of FIG. 1.

The column labeled “Csg pressurization” represents the step of pressurizing the casing 100, i.e. directing nitrogen into the tubing 101. The column labeled “water removal from csg” represents the step of removing water from the casing 100, i.e. by continuing to direct gas into the tubing 101.

The column labeled “water removal from tubing” represents the step of directing nitrogen into the casing 100, so that the nitrogen then flows upward through the tubing 101, and thereby displaces water from the tubing.

The column labeled “Nitrogen purge” represents the purging of nitrogen from the system, after the water has been removed, and before the well is ready to resume production.

The column labeled “NG production” represents the condition wherein the well is producing natural gas, in the normal course of activity.

Valve 6 connects the casing to one or more pressure gauges (not shown), and thereby allows measurement of various pressures in the well. Valve 6 is therefore kept open substantially at all times, as its function relates to measurement only. In the following description, valve 6 is not mentioned further, as it is assumed that this valve will remain open.

The method of the present invention can comprise the following steps.

First, valve 4 and 9 are opened. Nitrogen from membrane unit 102 flows through Hose B, through valve 4, and into the tubing 101. Because valves 3 and 7 are closed, the nitrogen cannot exit the well, and therefore the pressure in the well increases. This step is performed until a desired well pressure is reached.

Next, one continues to pump nitrogen down the well, through Hose B, while valves 7 and 9 are opened. Valve 3 is opened. Then valve 1 is opened slowly, to allow pressurized flow into Line A and Line B. This pressurized flow comes from the nitrogen that flows upward through the casing 100, and which brings water with it. This nitrogen and water will flow through valve 9 into the waste water storage tank. In this way, water is removed from the casing, and is conveyed to the tank 103.

As pressure in the lines decreases, one closes all valves except valve 4 (and except valve 6). One continues to pump nitrogen, through Hose B, into the tubing 101, so as to increase the gas pressure in the casing to a desired level.

The nitrogen unit is temporarily disconnected from the well bore, and valve 4 is closed. Valve 5 is opened, allowing nitrogen to flow through Hose A, charging the casing 100 until a desired pressure is reached.

Then, valves 2 and 3 are opened, and valve 1 is opened slowly. The pressurized gas in the well flows into Line A and Line B, while nitrogen continues to enter the casing through Hose A, to lift the remaining fluid in the well. Valve 7 is closed during this step. Since nitrogen is being injected, through Hose A, into the casing, the nitrogen eventually comes up through the tubing, and pushes out the water from the tubing. The gas and water mixture again flows through valve 9 and into the waste water storage tank 103.

When the remaining fluid has been removed, and the pressures have dropped, the nitrogen unit is shut off, and valve 5 is closed. One allows the well to flow naturally for about twenty minutes, although the time required may vary depending on the circumstances of each particular well. This step causes nitrogen in the lines to be purged.

After the nitrogen has been purged, the well can be returned to production. All valves are opened, except valves 5, 7, and 9 which are closed. This arrangement allows natural gas to flow out of the well, through Line A, and into the separation unit 104, and through outlet conduit 106.

Note that Hose A and Hose B, together with the various valves, comprise means for selectively directing nitrogen from the nitrogen-producing unit to either the casing or the tubing, depending on the settings of the valves.

The average time spent, in the process of the present invention, is usually about two or three hours, from start to finish. During the process, the pressure at the bottom of the well may range from about 600-1800 psi. The depth of the well may range from about 8,500 to 10,000 feet, and the tubing is landed from about 8,000 to 9,500 feet. The invention is not limited by the depth of the well. The well could be as shallow as about 400 feet, and could have a depth of more than 12,000 feet.

The method of the present invention has many benefits. As noted above, the method saves time, as the process may take only 2-3 hours per well. Moreover, all water removed from the well is fully contained, and is directly collected by piping into the on-site storage tank.

In the prior art swabbing process, the amount of water that can be removed is limited. Once the gas well starts to produce, the swabbing process must be terminated, due to safety concerns. Thus, the well typically will still have a significant amount of water. In the method of the present invention, by contrast, virtually all of the water in the well can be removed. The method of the present invention is a closed-loop process, and there is no danger of gas exposure while the process is operating. Because the present invention pushes the liquids and sands in the well tubing to the bottom of the well casing, the tubing becomes cleared of any material that tends to prevent the flow of fluids. Thus, the invention facilitates the flow of fluids up the tubing. Because the present method removes a greater proportion of the water in the well, the method does not need to be repeated as often as does the prior art process.

The method of the present invention does not harm the environment. Because the process is a closed-loop procedure, all fluid transfers are contained within piping. Fluids can be efficiently transferred from the well to the storage tank, without substantial risk of being spilled onto the ground.

The present invention has a further advantage that it makes it easier to keep the well tubing relatively clean, i.e. not clogged by sand, so that the flow of product gas is unimpeded. In the prior art swabbing process described above, the depth to which the cable can be lowered is limited by the cleanliness of the production tubing. If the tubing has an accumulation of sand, the cable may not be lowered to the desired level. As a result, the well bore may still contain significant amounts of water and sand.

The method of the present invention, by contrast, pushes liquids and sands in the production tubing to the bottom of the well casing, and into a non-productive section of the well known as the “rat hole”. This step is critical to ensure that the production tubing is cleared of any material that would prevent the flow of fluids, and subsequently gas. Displacement of the sand makes possible the flow of fluids up the production tubing. The reduced water and sand content of the well, compared to the result obtained from the prior art swabbing process, increases the time interval between required dewatering, and therefore allows the well to be used for a larger fraction of the time.

The present invention also has the advantage that it is safe. It is a closed-loop process, and all fluid transfers are contained within piping. Thus, personnel are not exposed to hydrocarbon gases, or to other fluids contained in the well.

As a result of the above advantages, a well which has been dewatered according to the present invention can be placed in operation more quickly, thereby increasing the production of the well. Over time, the level of production of gas is increased, and the amount of “down time” is reduced.

The invention may be modified in various ways. The exact arrangement of conduits and valves can be varied, to accomplish the major goal of the invention, i.e. the removal of water from a well by pressurized nitrogen. There are various ways of controlling the pressure of the fluid which flows to the waste water tank. Also, the nitrogen trapped in the casing may need to be purged by use of a choke valve. These modifications, and others which will be apparent to the reader skilled in the art, should be considered within the scope of the following claims. 

1. A method of removing water from a natural gas well, the well including a casing inserted into and extending above a well bore, the casing including a tubing disposed within the casing, the method comprising: a) directing nitrogen into an interior of the tubing, while maintaining pressure in the casing, so as to force water out of the casing, and conducting said water into a storage tank, b) directing nitrogen into a region between the casing and the tubing, while maintaining pressure in the tubing, so as to force water out of the tubing, and conducting said water into a storage tank, and c) purging nitrogen from the casing and the tubing.
 2. The method of claim 1, wherein step (a) is preceded by the step of pressurizing the casing.
 3. The method of claim 1, wherein steps (a) and (b) include operating a membrane-based unit for producing nitrogen, and directing said nitrogen into one of the interior of the tubing or the region between the casing and the tubing.
 4. The method of claim 3, wherein step (c) comprises interrupting a flow of nitrogen into the casing, and allowing natural gas to flow out of the well, so as to remove residual nitrogen from the casing and the tubing.
 5. A method of removing water from a natural gas well, comprising: a) non-cryogenically producing nitrogen at a site of a natural gas well, and b) directing said nitrogen into a casing located at the site of the well so as to displace water from the well.
 6. The method of claim 5, wherein step (b) comprises selectively directing said nitrogen into a tubing located within the casing, or into a space between the tubing and the casing.
 7. The method of claim 5, further comprising directing said water into a storage tank located at the site of the well.
 8. The method of claim 7, further comprising purging nitrogen from conduits at the site of the well and then resuming production of the well.
 9. Apparatus for removing water from a natural gas well, comprising: a) a non-cryogenic unit for producing nitrogen, the nitrogen-producing unit being located at a site of a natural gas well, b) a generally cylindrical casing positioned over the well and above ground level, c) a generally cylindrical tubing positioned inside the casing, and being aligned with a bore of the well, d) conduit means for selectively directing nitrogen from the nitrogen-producing unit to either a region between the casing and the tubing, or to a region within the tubing, and e) means for conveying water displaced from the well to a storage tank located at the site of the well.
 10. The apparatus of claim 9, further comprising a separation unit for removing water from a product natural gas stream, the separation unit being connected to receive natural gas produced by the well.
 11. The apparatus of claim 10, wherein the nitrogen-producing unit comprises a polymeric membrane for separation of air into components.
 12. The apparatus of claim 11, further comprising a conduit for directing water from the water separation unit to the storage tank. 